The present invention relates generally to an anti-skid brake control system for optimizing braking characteristics in an automotive brake system and preventing vehicle wheels from skidding. More particularly, the invention relates to a technique for avoiding malfunction in anti-skid control, which may possibly occur in various situation on which the vehicle is situated, especially while passing through a road having relatively high friction on the road surface.
In such anti-skid control system, anti-skid control is generally performed when the wheel is decelerated at a deceleration rate greater than a predetermined deceleration threshold. In the usual state, wheel deceleration greater than the deceleration threshold appears only when the vehicle is decelerated by application of the vehicular brake. However, in certain situations, it is possible to detect wheel deceleration greater than the deceleration threshold even when the brake is not applied. For example, when the road surface has a relatively high friction rate to provide good road tire traction, wheel speed may overrun beyond the wheel speed corresponding to the vehicle speed. In such situation, a relatively high rate of deceleration subsequent to overrunning appears to return the wheel speed to the speed corresponding to the vehicle speed. Such deceleration rate sometimes exceeds the deceleration threshold. This may occur when braking pressure in the vehicle brake system is decreased by the function of the anti-skid control system. As is per se well known, when wheel deceleration greater than the deceleration threshold is detected, the anti-skid control system become active to hold braking pressure constant until the wheel acceleration is increased across the deceleration threshold or the wheel slippage reaches a given slippage threshold set nearby the skidding criteria.
Therefore, when the anti-skid control system erroneously enters into operation in response to the wheel deceleration greater than the deceleration threshold appearing subsequent to overrunning of the wheel, the brake pressure is held constant decreased or at a completely released level to cause "no-braking state" while the brake is applied. This necessarily prolongates the braking distance and subjects the vehicle to serious danger.
In another possible situation, wheel deceleration greater than the deceleration threshold may be detected when "wheel-spinning" during abrupt acceleration of the vehicle. For instance, when wheelspinning during abrupt deceleration occurs, wheel speed is extraordinarily increased beyond the speed corresponding to the vehicle speed. When wheel-spinning terminates, the wheel speed is decreased to the speed corresponding to the vehicle speed. At this situation wheel deceleration is necessarily greater than the deceleration threshold. The anti-skid brake control system thus become active to feed a control signal to hold or decrease the brake pressure. This control signal may possibly be maintained to keep the brake system at the hold or release mode to disable increasing of braking pressure.